Cross-linked compositions, including copolymers of ethylene and vinyl acetate (EVA), are well-known in the art and have a wide variety of end uses. In particular, cross-linked EVA is especially suited for applications requiring superior thermal and mechanical performance, such as in wire and cable insulation material. EVA may be crosslinked using, for example, silanes, peroxides, and/or electron beam radiation. It is known in the art to partially crosslink an EVA copolymer using an organic peroxide. In such cases, crosslinking is typically initiated in an extrusion step and/or during a subsequent compounding step, in which additional components such as other polymers, fillers and/or additives are blended with the EVA copolymer (usually by a customer, rather than the producer, of the copolymer). Crosslinking may also take place after the EVA copolymer has been formed into a final product, such as, for example, by extrusion onto a wire or cable as a coating material and then heating to initiate crosslinking. In such situations, the composition is may be partially crosslinked prior to forming or extruding, and full crosslinking is then initiated on the wire or cable.
U.S. Pat. No. 5,182,072 discloses a process wherein an EVA copolymer and an organopolysiloxane are mixed at a temperature of at least 160° C. for a sufficient length of time to provide a melt index of from 0.05 to 8 g/10 min. Addition of an organic peroxide in an amount from about 0.001 to about 1 parts by weight is optional.
U.S. Pat. No. 5,194,516 discloses a process for increasing the viscosity of copolymers of either (a) ethylene and C1-C8 acrylates, or (b) ethylene, vinyl esters of C1-C4 carboxylic acids, and another comonomer, by treating the copolymer with a free radical-generating agent. Such treatment is said to reduce the melt index of the copolymer to 0.1-15 g/10 min while producing a copolymer product having a gel content of less than 3%.
U.S. Pat. No. 5,378,539 discloses a flame-retardant thermoplastic composition comprising a blend of ethylene copolymers, polyethylene, metal hydroxides, and optionally a coupling agent and a compatabilizing agent. The composition is mildly crosslinked with peroxide and a polyfunctional compound during blending.
U.S. Pat. No. 5,955,525 discloses an insulating formulation that is a mixture of vulcanizable halogen free polyolefins with low smoke emission and fire resistance characteristics formulated with a hydrated inorganic charge, an alkoxysilane, a curing agent, an additive such as a process assistance agent, and a combination of two antioxidants. The formulation is said to be suitable for use in compounds to insulate and cover cables and electric conductors.
WO 91/07761 discloses a cable coating composition comprising an ethylene-derived polymer having a melt index from 0.1 to 10 g/10 min, a filler, a crosslinking agent, and a stabilizer. The composition is extruded onto a conductor core prior to crosslinking and then crosslinked by activating the crosslinking agent.
WO 89/05328 discloses a process for producing an intermediate product for subsequent conversion to objects made of ethylene vinyl acetate. A peroxide is added to the product as a crosslinking agent.
JP 2005187497 A, JP 2003171421 A, and DE 3732722 A1 may also provide relevant background information in the field of crosslinked EVA copolymers.
A method has not been previously described for forming a crosslinked EVA copolymer that increases the molecular weight of the copolymer and maintains or increases the tensile strength while significantly lowering the melt index of the resulting partially crosslinked polymer. Such a method and the partially crosslinked copolymer that result from such a method are beneficial because the tensile strength and heat resistant properties of the copolymer are improved, while maintaining the flexibility and burning performance that exist due to the presence of vinyl acetate in the copolymer.